Rolling shutter or global shutter – which is best?
CMOS imaging sensors are at the heart of every industrial camera system, but they come in two variants – rolling shutter or global shutter. Each type of shutter has its advantages, but which one will work best for you depends on your application. So, let’s take a look at the differences between them to help you decide which one is going to work best in your machine vision system.
Rolling shutters
A CMOS imaging sensor contains many individual light sensors arranged in a grid (usually rectangular). This grid of sensors captures the image light and the camera converts this into a grid of pixels that make up the final digital image. With a rolling shutter, the image is captured one row of pixels at a time, moving across or down the grid to build up a complete image.
The advantage of this is that at any one time only a small amount of image data is captured, simplifying data processing and transmission. Because of that, rolling shutter CMOS sensors can be smaller and lower cost, resulting in cameras that tend to be cheaper and more compact than their global shutter counterparts. The disadvantage, however, is that with rolling shutters there is a time difference between the capture of the first and last row of pixels. This means that when the whole image is assembled, objects in the scene that are moving quickly may be distorted or blurred. The blades of the rotating propeller in the picture above show the distortion due to the rolling shutter.
Global shutters
A global shutter exposes all pixels in the sensor grid simultaneously, hence every pixel of the final digital image is captured at the same moment in time. This means that the image distortion due to moving objects/scenes seen with rolling shutters does not occur. Global shutter sensors are usually larger and more light-sensitive than rolling shutter sensors. However, global shutter technology is more expensive, and the maximum frame rate might be lower than that of a comparable rolling shutter sensor, especially with higher resolution sensors. A practical example, comparing the two shutter types, can be seen in this video showing car rims in motion.
Traditional applications for rolling shutter cameras and global shutter cameras
Due to the distortion of moving objects/scenes, rolling shutters are best used when imaging stationary or slow-moving subjects. They are ideal for capturing and processing high-resolution images in near real-time. In the context of machine vision, if parts can be brought to the camera, kept still and well lit, a rolling shutter camera with a CMOS sensor is going to be the optimal and most cost-effective option. Applications best suited to rolling shutter cameras include robotic pick and place, PCB inspection and microscopy.
Imaging parts moving at high speed is usually best achieved using a global shutter camera, which allows production lines to move faster and operate at increased efficiency. Applications benefiting from this technology include ANPR, high-speed production line inspection (e.g. pharmaceutical sector) and the automotive market. Higher demand means that more cameras with global shutter CMOS sensors are being produced. Higher volumes will encourage manufacturers to develop sensors/cameras with lower prices and improved specifications.
Cameras tailored to your application
Active Silicon offers a wide range of autofocus-zoom block cameras, including compact and cost-effective rolling shutter cameras and very light-sensitive global shutter cameras with powerful zoom. Global shutter Harrier cameras include models with 10x up to 36x optical zoom and several video output options e.g. LVDS, 3G/HD-SDI, USB, HDMI and Ethernet IP.
Rolling shutter cameras start with the cost-effective Harrier 10x AF-Zoom up to 55x zoom with 3G-SDI, USB/HDMI and Ethernet IP models, as well as 4K cameras.
Find the camera that’s right for your system – contact us now to discuss your requirements and we’ll advise on the ideal components for your machine vision system.
